Potentiometric sensors with ISFETs are used to determine electrochemical potentials, particularly as pH sensors. Such sensors are discussed in the article by P. Bergveld (“Thirty Years of ISFETOLOGY” in Sensors and Actuators B 88, 2003, P. 1-20), for example.
For an ISFET to work, it is essential that there is no leak current which can impact the measurement of the electrochemical potential. If a significant leak current is present, this implies that the potentiometric sensor is defective. For this reason, it is vital to monitor the sensor for leak current.
Common current monitoring systems with a shunt resistor in the measuring section cannot be considered, however, as currents in the range of 1 nA have to be detected which would either require a large resistance, which cannot be reconciled with the measurement, or a complex evaluation circuit which would be too expensive for the sensors in question.
In addition, a leak current can be detected using the counter-current principle. Here, a controlled current source is integrated in the circuit to be monitored which should impress a current that counteracts the leak current. An additional measuring device, which can determine whether the sum of the leak current and the counter current results in a value “zero”, controls the current source. The system can then determine to what extent a leak current is present on the basis of the control signal required. The counter-current principle is also not suitable for monitoring a potentiometric sensor because it involves increased power consumption, which—particularly for sensors with galvanically isolated, inductively coupled interfaces, as sold by the patent applicant under the name Memosens—is not readily available for potentiometric sensors. Similarly, a counter-current system involves increased costs which are not justifiable for potentiometric sensors.